Altmetric does track Google Books! All Google Books with ISBNs are tracked, not just ones that pertain to scholarly output.
How does it work?
When we see a Google Books mention in any of our sources, we try to identify the book by sending a query to the Google Books API. The Google Books API returns all of our requested metadata (book title, authors/editors, and so on), which we then display in Altmetric details pages.
If we use any metadata from Google Books, we attribute the data to Google within the details pages (by showing "powered by Google", as seen in the example below) and also within the Altmetric API output.